Simple beam scanning methods for low currents

NUCLEAR

INSTRUMENTS

AND

METHODS

94 (I971)

315-318;

©

NORTH-HOLLAND

PUBLISHING

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S I M P L E BEAM SCANNING M E T H O D S FOR L O W CURRENTS H. A R N O L D

Institute for Nuclear Physics Research (IKO), Amsterdam, The Netherlands Received 15 F e b r u a r y 1971 An i n s t r u m e n t for direct registration o f the position, the intensity and the energy distribution o f low intensity b e a m s for nuclear experiments is described. T h e e q u i p m e n t was tested for a charged particle and a g a m m a - r a y beam. It m a y even be used to scan the

intensity distribution o f n e u t r o n beams. T h e distributions are determined with respect to the s y m m e t r y axis o f the i n s t r u m e n t , which normally will be aligned with the ootical axis o f the beam t r a n s p o r t system.

1. Introduction

The intensity distribution is scanned by a plastic scintillation detector moving through the beam profile. An average current proportional to the integrated pulses of the scintillator feeds the ordinate of an oscilloscope, whereas the translation of the scintillator is coupled to the abscissa (see fig. I). For the energy scan a narrow gold foil is translated through the beam profile. At any position the spectrum of elastically scattered beam particles is detected by a lithium drifted silicon detector. From the width of the peak the energy spread can be derived, and from the possibly different positions of the peak (e.g. after a dispersive system) the local energy of the beam can be calculated.

Since both the intensity and the energy distributions are important parameters of the beam in a nuclear experiment one instrument has been designed to measure these two parameters shortly after each other. The aim of this instrument is to fill the gap between the method of measuring the current on a wire moved through the beam* or density measurement on photographic emulsions t (reasonable high currents) and the method to register the individual particles a piece by counter techniques or nuclear emulsion plates (very low currents). With the instrument here described, changes of the beam focussing elements can be checked immediately at any position along the beam system. This considerably facilitates adjusting the beam parameters. The two scanning functions of the instrument are separated. * C o m m e r c i a l l y available by High Voltage Eng. and Pr6citechnique D a u p h i n 6 , Grenoble, France. t H. A n n o n i et al., C e r n - R e p o r t 70-27 (1970).

2. The intensity scan

The translational motion of the scanning scintillator is obtained by the rotation of a double worm (leftand right-hand threads) of a large pitch, crossing each other at the same axle, and passing into each other at the outer ends of the spindle. An aluminum carriage supporting the scintillator is

Motor

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Fig. 1. Schematics o f the b e a m scanner.

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Fig. 2. Principle o f the double w o r m with the scintillator carriage. T h e gold target is in rest position. T h e Si(Li) detector is n o t seen in the u p p e r part o f the i n s t r u m e n t . T h e light guide has been r e m o v e d on the picture.

coupled to the worm with a pawl by which the rotation of the spindle is transformed into a periodic translation of constant speed except at the turning points (see fig. 2). The whole system is put into a chamber which can be coupled to a vacuumpipe. The worm is driven by a motor in air using a magnetic coupling through the wall of the vacuum box. By varying the voltage on the motor the translation speed of the scintillator can be adapted to the intensity of the beam to obtain sufficient output from the scintillator. The beamspot can be scanned with a maximum speed corresponding to 0.1 sec per translation. The motor can only be started or stopped when the carriage is in its end position on the spindle, in which position a magnetic reed contact is closed. Thus a single scan may be performed by switching on, immediately followed by switching off. This procedure may be useful and a memoscope is then usefully employed. When the carriage is at its end position on the spindle, a slit system, described later, protects the scintillator from the beam. This slit also cuts off information obtained during the not uniform motion near the endpoints.

The position of the scintillator is transferred to the abscissa of an oscilloscope by coupling the translation to the slider of a potentiometer. The tension on the slider, feeding the abscissa, is proportional to the position of the scintillator. A Schmitt-trigger, triggered on the minimum and maximum tensions on the slider generates a two-level voltage. The different levels correspond with the two directions of the periodic motion. For each direction of the motion the analog signals from the scintillator are added to the voltage

•I

Upper

Relativ

~ ~ - -

~

trace

Position

Fig. 3. T h e intensity distribution f r o m one o f the external b e a m s o f the I K O synchrocyclotron. T h e different results for lower a n d upper trace are due to different settings of one o f the focussing elements.

SIMPLE BEAM SCANNING METHODS FOR LOW CURRENTS concerned. In this way the scans in both directions are separated on the oscilloscope screen to eliminate possible hysteresis effects, resulting in a better resolution. When a single scan is performed both the output curves can correspond with different settings of the focussing parameters of the beam. This is seen in fig. 3 which shows a typical result for the intensity distribution of one of the external beams of the synchrocyclotron of the Institute for Nuclear Physics Research in Amsterdam. The analog signal is obtained by coupling the scintillator to a Philips 56AVP photomultiplier tube with a flexible light guide through the wall of the vacuum box. A reasonable compromise between minimum linearity and optimal amplification was achieved with the tube fed as shown in fig. 4. The value of the anode resister can be varied to adapt the amplification to the intensity of the scanned beam.

max voltag~ bct.w¢¢n dynodcs 164 V 0mA 164 V

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3. The energy scan

The energy distribution of a charged particle beam can only be measured with the instrument with the intensity scanning switched off. By rotating slowly a second screw spindle outside of the vacuum, an axle bearing a small selfsupporting gold target can be moved into the vacuum box through an O-ring sealing. At equidistant positions of the target (registrated by counting the number of complete rotations of the spindle) the spectrum of elastically scattered particles is measured with a lithium drifted silicon detector. This detector is mounted in such a way that its centre line intersects the symmetry axis of the apparatus under an angle of 20" and crosses the translation axis of the target perpendicularly (see fig. 2). Depending on the position of the target with respect to the symmetry axis particles are detected scattered over angles between 20 ° and 30 ° . The kinematical energy spread for protons within this interval is about 10 keV per MeV beam energy. A special slit system containing a defining slit and an anti-scattering one is used for the energy measurement. In the direction perpendicular to the plane through the translation and symmetry axes of the instrument the beam is defined by a slit system which avoids particles to be scattered on the carriage, and the detector to be hit directly by particles from the beam. The length of the slit in the translation direction is chosen such that both the scintillator for the intensity measurement and the target foil in their end positions on the spindles are protected by the slit material. The defining slit dimensions can be adapted to the requirements for counting rate and resolution. When the instrument is not in use for energy measurement, i.e. when the gold foil is in rest position at the end of the translation interval the Si(Li) detector is protected by a particle stopper to avoid radiation damage. After amplification in a charge amplifier the analog pulses of the Si(Li) detector are analyzed in a pulse height analyzer in the usual way.

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The author is indebted to Prof. D r . A . H . W a p s t r a and Dr. L. A. Ch. Koerts* for valuable discussions, to J. H. M. Bijleveld, A. Boucher and H. F. R. van Doornik for the design and realization of the mechanical part of the instrument and to J . T . van Es and B. van Randeraat for the design and construction of the electronical and electrical circuits.

acc

gl

Fig. 4. The bleeder feeding the 56AVP photomultiplier tube.

* Present address: Philips Research Lab., Eindhoven, The Netherlands.

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This work is part of the research program of the Institute for Nuclear Physics Research ([.K.O.), made possible by financial support from the Foundation for

Fundamental Research on Matter (F.O.M.) and the Netherlands Organization for the Advancement ot Pure Research (Z.W.O.).